1. Field of the Invention
The present invention relates to power control, and more particularly to a power controller linked with a commercial power line and power source apparatus using such a power controller.
2. Description of the Related Art
In recent years, concern about environments and energy has rapidly grown with regard to such problems as warming of the earth due to large amount of CO.sub.2 exhaust, radioactive contamination caused by accidents in nuclear power plants, and disposal of nuclear wastes. Correspondingly, wind power generation, geothermal power generation, solar cells, etc. have been studied with deeper interest. In these situations, solar cells and the like are especially expected as a regenerable, inexhaustible and clean energy source from all over the world. However, additional assistance is required to enable a power source such as a solar cell source, from which energy can be taken out only in the daytime under the sun, to be used as a stable energy source.
The simplest assistance is provided by storing output energy from a power source such as solar cells in a storage battery, and taking out the energy stored in the storage battery in the nighttime or under other circumstances in which no energy is output from the power source such as solar cells. Realizing such energy storage, however, requires a storage battery of considerable capacity. This makes the entire power generation system expensive. Also, service and maintenance of a steerage battery is troublesome. For these reasons, the above method using a storage battery is often employed in those remote places and isolated islands where power consumption is relatively small and other power sources are hard to obtain.
On the other hand, another method is known for realizing the above assistance, in which output of a power source such as wind power generation, geothermal power generation, solar cells, etc. is subjected to power control using a line linking inverter, and the controlled power is linked to a commercial AC power line (hereinafter referred to also as a line power source) for use in daily life so that the commercial power line may be utilized instead of the above storage battery. This method is regarded as the most likely one for more development and widespread use of a power source apparatus such as a solar cell system, because it requires neither an expensive steerage battery nor additional area for installation if solar cells and the like are installed on a roof.
One example of the above type power source apparatus is shown in FIG. 7. A solar cell array 1 is formed by combining a plurality of solar cells, one kind of power source, in series--parallel connection. DC power output from the solar cell array 1 is converted into AC power through a line linking inverter 2 including a DC/AC conversion unit 21 and a control unit 3. The AC power is further converted from a single-phase two-wire form into a single-phase three-wire form by an insulating transformer, and is supplied to various general usage loads 41 and 42. A single-phase three-wire commercial power line 5 is connected to the load 41, the load 42 and the line linking inverter 2. (Herein, a single-phase three-wire line corresponds to a two-phase line when considered from both sides of a neutral wire).
In the power source apparatus thus constructed, when the power consumed by both the load 41 and the load 42 is less than the power output from the line linking inverter 2, surplus power is supplied to the commercial power line 5 to produce the so-called reverse tidal power. When the power consumed by both the load 41 and the load 42 is larger than the power output from the line linking inverter 2, a shortage of power is compensated for by the power supplied from the commercial power line 5. By this construction, therefore, any of various type power source apparatus can be used as power generation equipment which provides energy in a stable manner.
Meanwhile, electric power is usually supplied to individual residences through a single-phase low-voltage distribution line. The single-phase low-voltage distribution line is divided into two types; i.e., a single-phase three-wire line and a single-phase two-wire line. Given the rated voltage of a single-phase two-wire line as Vn, a single-phase three-wire line can supply two levels of voltage, Vn and 2.times. Vn. In Japan, for example, because Vn=100 V, it can supply 100 V and 200 V. In the USA, it can supply 120 V and 240 V because Vn=120 V. Electric appliances for use in individual residences, particularly, those appliances stich as air conditioners which consume a large amount of power, have become increasingly popular. Single-phase three-wire distribution lines are very suitable to support a supply capacity accommodating such an increase in power demand. It is therefore believed that single-phase three-wire distribution lines will be more and more prevalent in future.
Generally, it is said that in order to safely link a power source apparatus to a power line, output of the power source apparatus should desirably match the power line in a distribution mode.
For that reason, as explained above with reference to FIG. 7, the transformer is connected to the output of the single-phase three-wire line linking inverter 2 for conversion into a single-phase three-wire output. Thus, this method has been regarded to be very advantageous in easily obtaining a single-phase three-wire output at the relatively small cost.
However, the inventor has found basic problems related to single-phase three-wire distribution lines are encountered when bringing the above-explained line linking system of reverse tidal power type into operation; i.e., problem of load balance and problem of protection loss of one phase.
(1) Problem of load balance
To enable consumers of electricity to safely use electric appliances, the supply voltage from electric power companies is set to fall within a certain range. (In Japan, for example, the supply voltage is held in the range of95 to 107 V).
Since distribution lines have impedance, the voltage on the power transmitting side is usually higher than that on the power receiving side. In the case of a residence contracted for capacity of 3 KW, for example, the diameter of the distribution line is selected so that the supply voltage is in the stipulated range even when power is consumed at 30 A, and a voltage drop is considered in design so that the rated voltage of a transformer on a utility pole becomes 105 V.
In a single-phase three-wire line, however, the supply voltage may depart from the above stipulated range due to load imbalance. This often occurs because the line voltage varies depending on time to a larger extent than expected. FIG. 6 shows such time dependent variations of the line voltage in Japan. In FIG. 6, one phase (R-phase) represents a load current of 12 A and the other phase (T-phase) represents a load current of 2 A. At points A in FIG. 6, the voltage on the side consuming a smaller current with respect to a neutral wire exceeds 107 V, the upper limit voltage stipulated in Japan, and rises to 108 V. This problem of load balance also occurs in the USA. Ordinarily, electric appliances are able to operate at a voltage maximally exceeding the rated voltage by 10% and no practical troubles occur in power consumption for electric appliances. Such an event is however never said to be satisfactory from the side of distribution lines, because it apparently increases the loss. If a balancer as one kind of autotransformer is used, balance could be obtained between both the phases. But, using such a balancer with a large-capacity is expensive and costly. Further, a balancer is less likely to be used since electric appliances can be used without troubles in the absence of balancers.
On the other hand, the above-mentioned imbalance between both the phases gives rise to a severe disadvantage from the standpoint of linking a power source apparatus to a commercial line system.
Usually, in the event an excessive voltage rise such as an abnormal voltage occurs, the line linking system is stopped by an overvoltage relay for protection of the line linking system.
Therefore, when the voltage on one phase side increases excessively due to imbalance between both phase sides, the line linking system is entirely stopped, meaning that if there is an allowance to accommodate reverse tidal power on the other phase side, supply of such reverse tidal power must be stopped. This leads to a large loss from the standpoint of effectively utilizing the power from the power source apparatus.
In a solar power generation system wherein a transformer is connected to an output of a single-phase two-wire line linking inverter for conversion into a single-phase three-wire output, it seems that the transformer serves as a balancer and the problem of load balance does not exist. However, if the transformer itself is not balanced, an imbalance between both phase sides occurs. Because the line linking system of FIG. 7 includes the single power conversion unit which can be controlled, using a completely balanced transformer is the only one effective method to eliminate such imbalance.
(2) Problem of protection for lack of one phase
This problem of protection for loss of one phase is more intricate and more difficult to cope with than the above problem of load balance. Here, the term "loss of one phase" implies an event wherein one phase side of a single-phase three-wire line (i.e., an electromotive force in different phase at the same frequency) is placed in an open, power-failed state by an accident such as breaking of the line distribution line. In that event, there is a fear that loads concentrated on the other remaining phase will cause an overcurrent, and the power source apparatus should be stopped from the standpoint of safety.
In the line linking system of FIG. 7, however, the presence of the aforesaid transformer makes it difficult to stop the power source apparatus by detecting the above event. More specifically, if prior to start-up of the line linking system, the overcurrent state could be detected by providing a breaker between the transformer and the output terminal, and disconnecting the transformer at the breaker. But, during operation of the power source apparatus, the overcurrent state is difficult to detect. This is because the transformer serves as a balancer and closely couples both phases each other. While the possibility of the line linking inverter breaking under operation in the overcurrent state is small or substantially zero, the overcurrent state is nothing but an abnormal operating condition and the power source system should be desirably stopped.